Documentation/remote-helpers: explain capabilities first

Documentation/remote-helpers: explain capabilities first

The current remote helper documentation is from the perspective of
git, so to speak: it presents a full menu of commands for a person
invoking a remote helper to choose from.  In practice, that's less
useful than it could be, since the daunted novice remote-helper author
probably just wanted a list of commands needs to implement to get
started.  So preface the command list with an overview of each
capability, its purpose, and what commands it requires.

As a side effect, this makes it a little clearer that git doesn't
choose arbitrary commands to run, even if the remote helper advertises
all capabilities --- instead, there are well defined command sequences
for various tasks.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>